Well bores are drilled to locate and produce hydrocarbons from geologic formations. A down hole drilling tool with a bit at an end thereof is advanced into the geologic formation to form a well bore. As the drilling tool is advanced, a drilling mud is pumped through the drilling tool and out the drilling tool to cool the drilling tool and carry away cuttings. The drilling mud additionally forms a mud cake that lines the well bore.
Formation evaluation often requires that fluid from the formation be drawn into the down hole tool for testing and/or sampling. Various devices, such as probes, are extended from the down hole tool to establish fluid communication with the formation surrounding the well bore and to draw fluid into the down hole tool. A typical probe is a circular element extended from the down hole tool and positioned against the sidewall of the well bore. A rubber packer at the end of the probe is used to create a seal with the wall of the well bore. Another device used to form a seal with the well bore is referred to as a dual packer. With a dual packer, two elastomeric rings expand radially about the tool to isolate a portion of the well bore there between. The rings form a seal with the well bore wall and permit fluid to be drawn into the isolated portion of the well bore and into an inlet in the down hole tool.
The mud cake lining the well bore is often useful in assisting the probe and/or dual packers in making the seal with the well bore wall. Once the seal is made, fluid from the formation is drawn into the down hole tool through an inlet by lowering the pressure in the down hole tool. Examples of probes and/or packers used in down hole tools are described in U.S. Pat. Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045; 6,609,568 and 6,719,049 and U.S. Patent Application No. 2004/0000433.
Formation evaluation is typically performed on fluids drawn into the down hole tool. Techniques currently exist for performing various measurements, pretests and/or sample collection of fluids that enter the down hole tool.
Fluid passing through the down hole tool may be tested to determine various down hole parameters or properties. The thermophysical properties of hydrocarbon reservoir fluids, such as viscosity, density and phase behavior of the fluid at reservoir conditions, may be used to evaluate potential reserves, determine flow in porous media and design completion, separation, treating, and metering systems, among others.
Formation evaluation may also be performed on gasses entering the drilling mud. One technique for performing this evaluation is known in the art as gas phase chromatography. Gas phase chromatography is a technique which may be used for the separation and quantification of mud gas components. Mud gas analysis using gas phase chromatography may allow monitoring of the drilling process for safety and performing a pre-evaluation of the type of fluids encountered in drilled formations. To extract gases from the drilling fluid, a gas extractor (often called degasser) such as the Geoservices Extractor, U.S. Pat. No. 7,032,444 may be used. Alternatively, selective membranes and sonication, have been used to release gas from the drilling fluid. After extraction, the mud gases may be transported and analyzed directly in a mud logging unit. It may be desirable to perform a qualitative and/or quantitative continuous compositional or isotopic analysis on fluids involved in mud gas analysis to be able to characterize the hydrocarbons present in the drilled formations versus depth. The more measurements performed, the better the level of resolution of gas events described by the mud logging services.
In the past few years, continuous real time (CRT) logging of isotopic compositions (typically expressed as delta—δ, e.g. δ13C) of methane extracted from drilling mud during drilling operation was introduced as an additional tool for real time geochemical interpretations of the hydrocarbon system (Jones et al. 2005, Breviere et al. 2008, Breviere et al. 2009). Isotopic composition of methane (i.e. δ13C or δ2H also written as δD) as well as other gases have been used for several decades now for such interpretations, e.g. Bernard et al. 1978, Schoell 1983, Berner & Faber 1988, Whiticar 1999, etc.). Real time isotope logging as well as spot sampling (e.g. Isotubes) introduce many challenges for obtaining isotopic composition of gases representative of the formation gas or results of PVT-sample quality (results comparable to fluid samples taken from reservoir at reservoir pressure and temperature conditions). Main challenges include contamination by hydrocarbons generated via bit metamorphism (Faber et al. 1988, Wenger et al. 2009) and by gases recycled in mud during drilling operation. Whether isotopically unchanged or fractionated by surface degassing, the recycled gas will mix with formation gas and to certain extent affect the composition of the measured gas that is coming out with the mud.
The recycling issue for molecular gas composition has been addressed by quantitatively analyzing mud gas coming out (gas OUT) of bore hole and gas from mud that is being injected. Synchronization of these gases and subtraction of the gas going into the borehole (gas IN) provides quantitative formation gas composition of methane, ethane, propane, iso-butane, n-butane, iso-pentane, and n-pentane, i.e. in Fluid Logging and Analysis in Real-time (FLAIR) technology (Duriez et al. 2002, Breviere and Evrard 2006, Frechin and Breviere 2006, McKinney et al 2007). Currently there is no correction to account for recycling being applied in isotope logging.
Mud degassing, in general, will preferentially leave heavier gas species (e.g. 13C-enriched) retained in the mud to the degree controlled by i) α and ii) fraction of gas species not-degassed. The less gas remaining in the mud the more 13C-enriched the gas will be according to the kinetic isotopic fractionation α (in this case it is an open system, where liberated gas is removed from the system as it escapes to the atmosphere). Conversely, mud gas fraction extracted for analysis in a mud gas extractor will likely by slightly 13C-depleted, as such extraction is never complete and the liberated gas will be 12C-enriched. The degree of gas retention and recycling in mud as well as the isotopic fractionation during mud degassing can vary with mud and atmospheric conditions (e.g. temperature), time each portion of mud spends at the surface conditions exposed to atmosphere, type of mud (e.g. water based mud or oil based mud), type of additives (e.g. with sorption affinity for hydrocarbon gases, such as lignite), mud salinity, mud density, intensity of mud agitation and/or centrifuging on the surface, type of shale shakers, etc. Correction for extraction efficiency coefficient (EEC) for gaseous hydrocarbons has been successfully applied by using isobaric and isothermal conditions in a constant volume-degasser (Frechin and Breviere 2006).
Recently, continuous real-time isotope logging has been used to obtain δ13C measurements from the mud gas that is being extracted from mud continuously flowing out of the wellbore during drilling. (See Jones et al. 2005, Breviere et al. 2008, Breviere et al. 2009). A development of applying Cavity Ring Down Spectroscopy to δ13C1 analysis was introduced a few years earlier (Uehara et al. 2001). This measurement provides isotopic composition of a mixture of freshly drilled formation gas, recycled gas, and bit-metamorphism gases (Faber et al. 1988, Wenger et al. 2009), and in specific conditions (e.g. underbalanced mud weight) intrusions of gas from already drilled shallower formations.
However, the readings obtained by the conventional techniques suffer from many drawbacks including inaccuracy of reading the isotopic composition of gas due to the impact of mixing formation gas with gas recycled in the drilling mud.